Signal selection in radio receivers



1941- H. A. 'FAIRHURST ,268,505

SIGNAL SELECTION IN RADIO RECEIVERS Filed June 25, 1940 2 Sheets-Sheet lFIGI.

Dec. 30, 1941.

-l. A. FAIRHURST 2,268,505 SIGNAL SELECTION IN RADIO RECEIVERS FiledJune 25. 1940 2 Sheets-Sheet 2 A T TOR/V5 Y Patented Dec. 30, 1941SIGNAL SELECTION IN RADIO RECEIVERS Harold Alfred Fairhurst, WelwynGarden City, England, assignor to Murphy Radio Limited,

London, England Application June 25, 1940, Serial No. 342,315 In GreatBritain June 23, 1939 8 Claims.

This invention relates to radio receiving apparatus. Its purpose is toenable the suppression of undesired signals of short duration withoutmaterially affecting the transmission along the signal channel of thesignals it is intended.

to pass.

A particular instance of this purpose is the suppression in a radiotelephone receiver of the type of interference caused by the ignitionsystems of motor cars, that is to say, interference of the nature ofimpulses of less duration than the period of the highest audible note orharmonic it is desired to receive, these impulses often recurring at anaudio frequency. The invention is particularly concerned with thisproblem as it arises in receivers working on very short wavelengths suchas the sound receivers of television apparatus.

The invention rests on the differentiation of the undesired signal ornoise from wanted signals by the short duration of its componentimpulses, and aims to remove any such noise by shunting brief pulseswhich rise above the instantaneous level of the wanted signal.

It is common practice to reduce the effects of such interference inradio receivers by the use of some form of limiter circuit, designed tolimit to a fixed maximum value the output produced by all signalsapplied to it.

Such limiting circuits applied to telephony receivers must be soadjusted as. to pass the wanted modulation without distortion at alllevels up to that corresponding to 100% modulation of the carrier wave.Hence they can limit noise impulses only to a corresponding amplitude.Since the average level of modulation in normal broadcast transmissionis of the order of the interference let through will often considerablyexceed the instantaneous amplitude of the wanted signal.

A principal object of the present invention is to suppress interferenceimpulses of the kind above described without the loss of wanted signalthat must inevitably take place along a normal conductive limiter shunt.

In a radiotelephony receiver according to the present invention, thebias applied to the limiter, which determines the level to which theoutput signal is limited, is an independent bias (as distinct from aself-bias or autobias) which includes an audio frequency componentadjusted to substantial equality of amplitude with the signal at thepoint at which the limiter is branched from the signal channel so thatthe limiter is non-conductive to wanted signals free of interferencepulses, and this bias voltage is applied to the 1imiter through circuitswhich have a slight delaying effect. The difference in phase thusbrought about should be a small fraction of the periodic time of thehighest frequency oscillations it is desired to deal with, but shouldpreferably be not less than the-duration of an interfering noise pulse.

The characteristic of short duration of the component impulses of noise,on which the differentiation depends, must be preserved up to theaudio-frequency stage in which elimination of noise is effected; andtherefore the preceding radio frequency and intermediate frequencycircuits must respond over a Wave band of sufiicient width for thispurpose. The band width should be as large as is consistent withreasonable amplification and other competing considerations, and usuallyat least five times as wide as is called for by the highest audiofrequency to be reproduced. A convenient form of limiter, suitable forthe purpose of this invention, consists of a simple diode shunted acrossthe input of an audio-frequency stage. The signals to be differentiatedare applied to both electrodes of the diode, to the anode directly, tothe cathode.

with slight delay. The cathode input may be taken from theoutput of thesucceeding audio frequency stage in which case the delay circuit may bein either the input or the output circuit of that stage; or it may betaken from a triode preferably used as a cathode follower, forming abranch from the signal channel in which case the delay circuit may be ineither the input or output circuit of the triode. The delaying circuitmay be built up of series resistances and shunt condensers, orinductances may be used.

Typical circuits embodying the invention are illustrated in theaccompanying drawings.

Figure 1 shows the diode cathode fed from the output of the succeedingaudio frequency stage.

Figure 2 shows a modification of the scheme of Figure 1 providing fordesired tone compensation.

Figure 3 shows the diode cathode fed from a cathode-follower triodeforming a branch from the signal channel.

Figure 4 shOWs a modification of the scheme of Figure 3 in respect ofthe means for applying the appropriate-D. C. voltage to the anode ofthediode.

Figure 5 shows a further modification of this scheme in respect of theinput to the cathodefollower triode.

For the purpose of explaining the attainment of the principal object ofthe invention it may be assumed that the input terminals l in eachfigure receive audio frequency energy from a detector D or audiofrequency amplifier of any usual type and supply it to the control gridof an output valve 2 from which a loud speaker, not shown, is fed inusual manner, for instance through the transformer 3 of Figuresliand 2.

For the purpose of the invention the input circuit of the valve 2 isshunted by a diode 4. To the cathode of this diode there is applied abias comprising a D. 0. component intended to bring the diode to theverge of conducting and an audio frequency component intended to makethe cathode potential closely follow the potential of the anode for allsignals it is desired to,re-

ceive.

The signal input is applied to the anode of diode 4 and to the controlgrid of valve 2 through a condenser 5 and resistance 6. The grid leak lof valve 2 is returned to a tapping on the condenser-shunted cathoderesistance 8 by which the valve is self-biased. D. C. bias for the diode4 may conveniently be taken from a potentiometer 9 bridged across thecathode resistance 8, the tapped part of the potentiometer being shuntedby a condenser II and connected in series with the secondary winding oftransformer 3 between chassis and the cathode of diode 4. The connectionis not direct but is made through a delaying network shown as built upof resistances l2 and condensers I3. The amplitude of the audiofrequency voltage so applied to the cathode is adjusted to be equal tothe amplitude of the voltage applied to the anode of the diode; itsphase is shifted slightly by the delaying network; the delay should benot less than the expected duration of the short impulses it is desiredto obliterate.

. When normal signals Within the desired range of modulation frequenciesreach the anode of the diode 4 they are applied nearly simultaneously tothe cathode also and the cathode remains non-conducting. These signalstherefore pass along the signal channel without appreciable distortion.Short impulses of interference, on the other hand, whose duration issmall compared with the time period of the highest audio frequencies itis desired to pass through the signal channel, will reach a sufiicientvalue upon the anode of the diode to render the anode conductive beforethey can begin to appear on the cathode.

Such impulses are therefore shunted and do not reach the grid of valve2. Even should a second interfering impulse reach the anode of the diodeat the instant when the first, delayed by the network [2, I3, would(save for the shunting effect) have reached the cathode it will stillnot pass because the first impulse having been bye-passed to chassisdoes not reach the oathode at all.

It is commonly desired in radio telephony receivers to make provision inthe input of the output valve for reducing the bass response because itis likely to be unduly accentuated by loud speaker resonances. If anysuch compensation were attempted with the circuit of Figure 1, byplacing a series condenser and shunt resistance in the grid circuit ofvalve 2, its effect would be to produce an appreciable phasedisplacement between voltages of low audio frequency applied to theanode of the diode and the corresponding voltages applied to theoathode, with the result that the diode would exercise its shuntingeffect and cause distortion of the desired signal.

If such reduction of the bass is attempted, therefore, in the input tothe grid of valve 2, it must be compensated by a correspondingenhancement of the bass in the voltage applied to the cathode of thediode 4. This is illustrated in Figure 2 where a condenser I4 andresistance [5 in the grid circuit of valve 2 cause a reduction of thebass in the signals transmitted through the transformer 3, while theresistance I6 and condenser l! in the cathode circuit of the diode 4cause a compensating increase of the bass in the voltage applied to thecathode.

Additional turns would be needed on the secondary of transformer 3 toallow for the voltage drop in this compensating network.

It will be apparent that the compensating network IG, I1 is essentiallyof the same nature as the delaying network l2, l3, and therefore, asindicated by Figure 2, a single network may be designed to serve bothpurposes.

It is, however, diflicult to adjust the phase relationship between thevoltages on the cathode and anode of the diode in this circuit,especially for the lower audio frequencies. It is therefore preferableto supply the. cathode of the diode from an additional valve branchedfrom the signal channel. This is shown in Figure 3. The valve 18, shownas acting as a cathode follower, is fed through the condenser l9 fromthe same point as the diode 4; its grid leak 2| is returned to asuitable point on its cathode resistance 22. The input to the valve istherefore unaffected by the bass-reducing circuit I4, l5 of the valve 2.The resistances 6, l are adjusted to produce the same audio frequencyvoltage on the anode of the diode 4 as appears on the cathode of valvel8. The requisite D. C. bias for the diode anode is got by adjusting itsanode potential in relation to the potential of the cathode of valve I 8by a potentiometer 23. The requisite delay in the voltages fed to thediode cathode is obtained from a delaying circuit shown, in thisinstance, as consisting of series inductances 24, coupled in pairs, andshunt condensers 25 forming a low pass filter completed by a terminatingresistance 26. Such a circuit can be designed to give a sharp cut-off ata frequency above any desired audio frequency and below the eifectivefrequency of the interference.

As the voltage across the diode under optimum conditions is nearly zerothe circuit may be simplified as shown in Figure 4 by the omission ofthe potentiometer 23, the voltage on the oathode of the cathode followervalve [8 being applied instead to the anode of the diode 4 through asuitable smoothing circuit comprising resistances 21 and capacitance 28.

It will be noted that in the circuits of Figures 3 and 4 all noiseimpulses applied to the anode of the diode 4 and the grid of valve 2will appear, after the delay imposed by the network 24, 25, 26, upon thecathode also. If, therefore, a second noise impulse should follow thefirst at an interval approximately corresponding with this delay thesecond impulse might appear on the anode of the diode simultaneouslywith the appearance of the first impulse on the cathode, and would thennot be obliterated.

This disadvantage may be obviated by feeding the grid of valve l8 asWell as the anode of the diode 4 from the junction of the resistances Band 21 as shown in Figure 5. This will make the audio frequency voltageon the cathode of the diode slightly less than that on the anode, but ifthe cathode load is made considerable the difference can be reduced tonegligible proportions. The time constant of the input circuit 5, 6 issuch that the circuit passes low frequencies undistorted. Since anydelay greater than the duration of an interfering pulse is suflicient toprevent the building up of the pulse upon the anode of the diode and thegrid of valve I8, the

value of the time constant of the delay network 24, 25 is not critical,and may be designed with reference to the frequency response desired inthe output.

While the circuits above described are primarily designed for theelimination of interference of the kind first above noted from telephonysignals it will be apparent that they can be made to separate any shortimpulses from longer impulses. For example such a circuit as is shown inFigure 3 may be designed to eliminate line synchronising impulses from amixture of line and frame synchronising impulses in televisionreception. The grid of the valve of the usual saw-tooth generator issupplied from the anode of the diode 4, and the delay circuit 24, 25 isdesigned to produce delay exceeding the duration of a line synchronisingimpulse, but less than the duration of a frame synchronising impulse;thus the latter but not the former will reach the grid of the dis-chargevalve.

I claim:

1. In a radio receiver the combination with carrier frequency circuitshaving a width of response in excess of that needed for reproduction ofthe highest audio frequency desired and a rectifier, of an audiofrequency amplifier stage fed through said rectifier from said carrierfrequency circuits, a diode shunting the input circuit of said audiofrequency amplifier stage, and means for applying directly to the anodeof said diode and through a delaying circuit to its cathode signalvoltages adjusted to substantial equality of amplitude, said delayingcircuit introducing delay approximately equal to the duration ofexpected interfering pulses.

2. In a radio receiver, means for separating signals of long durationfrom other signals of short duration comprising an amplifier including agrid-controlled thermionic tube to which said signals are fed, a limitercircuit fed in parallel with the control grid of said amplifier, andmeans for introducing into said limiter circuit an audiofrequency biasderived from and varying with the signals and adjusted to substantialequality of amplitude with the signal fed to the limiter circuit, saidmeans including a delaying circuit which retards said bias relatively tothe signals by the duration of the signals of short duration.

3. In a radio receiver the combination with carrier frequency circuitshaving a width of response large compared with the range of audiofrequencies to be reproduced, a rectifier supplied from said carrierfrequency circuits, and an amplifier including a grid-controlledthermionic tube fed from said rectifier, of a diode having its anodealso fed from said rectifier, a tone-adjusting circuit included in theinput circuit of said amplifier, and means for applying to the cathodeof said diode a bias derived from the output of said amplifier, saidmeans including a delaying and tone adjusting network to retard saidbias relatively to the input to the diode and to compensate for thetone-adjustment effected by the tone-adjusting circuit.

4. In a radio receiver the combination with a signal channel comprisingcarrier frequency circuits having a width of response large comparedwith the range of audio frequencies to be reproduced, a rectifiersupplied from said carrier frequency circuits, and an amplifierincluding a grid-controlled thermionic tube fed from the said rectifier,of a diode having its anode also fed from said rectifier, a thermionictube fed from said rectifier as a branch from said signal channel, andmeans for applying to the cathode of said diode a bias derived from thecathode circuit of said branched thermionic tube, said means including adelaying network which retards said bias relatively to the input to thediode anode by an interval less than the periodic time of the highestaudio frequencies to be reproduced.

5. In a radio receiver the combination with carrier frequency circuitshaving a width of response large compared with the range of audiofrequencies to be reproduced, a rectifier supplied from said carrierfrequency circuits, and an amplifier including a grid-controlledthermionic tube fed from the said rectifier, of a diode having its anodealso fed from said rectifier, a cathode-follower tube fed from saidrectifier, and means for applying to the cathode of said diode a biasderived from the cathode circuit of said cathode-follower tube, saidmeans including a delaying network which retards said bias relatively tothe input to the diode anode by an interval less than the periodic timeof the highest audio frequencies to be reproduced.

6. In a radio receiver the combination with carrier frequency circuitshaving a width of response large compared with the range of audiofrequencies to be reproduced, a rectifier supplied from said carrierfrequency circuits, and an amplifier including a grid-controlledthermionic tube fed from the said rectifier, of a diode having its anodealso fed from said rectifier, a cathode-follower tube fed from the anodeof said diode, and means for applying to the cathode of said diode abias derived from the cathode circuit of said cathode-follower tube,said means including a low-pass filter adapted to cut off above thehighest audio frequency to be reproduced.

7. In a radio receiver, means for separating signals of long durationfrom other signals of short duration comprising an amplifier to whichsaid signals are fed, a limiter circuit connected in parallel with theinput circuit of said amplifier, a source of bias independent of theconductivity of said limiter circuit for deriving from the signal andapplying to said limiter circuit an audio frequency component adjustedto substantial equality with the signal at the point at which thelimiter circuit is branched from the signal channel, and delaying meansby which said bias is retarded relatively to the signal.

8. A radio telephony receiver having in combination with an audiofrequency amplifier included in the signal channel, a diode connected inparallel with the input circuit of said amplifier, and a bias circuitindependent of the conductivity of the diode circuit for deriving fromthe signal and applying to the cathode of said diode a bias ofaudio-frequency adjusted to substantial equality of amplitude with thesignal voltage applied to the anode of the diode thereby rendering saiddiode non-conductive for wanted signals, said bias circuit including adelaying network which retards the bias relatively to the signal by afraction of the periodic time of the highest audio frequency to bereproduced.

HAROLD ALFRED FAIRHURST.

